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In February, Olympic skiers such as Julia Mancuso, Ted Ligety, Marco Sullivan and Scott Macartney will race down Vancouver's Whistler Mountain at speeds of up to 90 miles an hour. Paul Doherty, senior scientist at the Exploratorium in San Francisco, and Sam Colbeck, a retired scientist from the U.S. Army Cold Regions lab, explain the physics of this downhill thrill ride.